The present invention relates to a group and method for laying and burying pipelines at the seafloor.
In laying pipelines at the seafloor it is also desirable to bury them in order to protect them from specific local factors such as fishing, navigation, streams, wave-motion, natural events and so forth.
On the basis of these factors and depending on the morphology and bathymetry of the seafloor, in addition to deciding upon the necessity of burying a partial tract or the whole length of a pipeline, the characteristics of the cavity, i.e. the digging depth, the entity and gradation of the covering, are also established.
In recent years, compatibly with the economical aspect implied in the burying operations of pipelines at the seafloor, this operation has become standard in sub-coastline tracts, also due to the imposition of regulations.
There are, in particular, three methods for laying and burying pipelines in the seafloor:                pre-trenching, i.e. the preliminary digging and treatment of the trench destined for receiving the pipeline, before the laying operation of the same;        co-trenching during the launching and laying of the pipeline; and        post-trenching after the laying of the pipeline, i.e. burying the pipeline by removing the underlying material, after being launched, hauled or placed in the desired position.        
The pre-trenching methods are preferably applied in the case of compact seafloors, consisting of cemented sediments or lithifications or rocks. The co-trenching and post-trenching methods, on the contrary, are particularly suitable for seafloors covered by a mantle of sediments having a thickness greater than the depth required for the excavation, or consisting of non-lithified or non-cemented materials or easily breakable materials.
The pre-trenching methods require a much larger excavation than the pipeline dimension, as the latter is laid subsequently, or a particularly precise laying technique, called guided laying.
The co-trenching methods require the concurrence of two types of equipment, launching and digging equipment, and the synchronism of the two operations making the procedure difficult to effect.
The post-trenching methods can be effected after laying the pipeline, as they do not require the contemporaneous presence of two different naval means.
The co-trenching and post-trenching methods have the disadvantage of a possible deformation or damage to the pipes during the digging phase, as shown in FIG. 1.
During the digging, in fact, the pipelines are in a so-called “span” configuration, i.e. suspended between the area of the seafloor already excavated and the portion of seafloor not yet emptied, in particular in correspondence with the immediate position of the trenching machine.
In this configuration, if the difference in level between the unexcavated and excavated portion is considerable, the pipelines are subjected to stress which could cause the deformation or even collapse of the same.
Consequently, the post-trenching, and co-trenching excavations are currently effected with successive small-depth passages in order to reduce the span which is created between the digging machine and the seafloor already excavated, to the minimum.
The necessity of various digging passages in any case causes a lengthening of the overall operating period necessary in the co-trenching and post-trenching methods.